The date of the transfer is uncertain. The National Park Service will not round up bison from inside the park for the relocation, but only those animals that leave the park in winter in search of food, Yellowstone spokesman Dan Hottle said Wednesday.

Officials were trying to determine if there are enough bison outside the park's boundaries to begin capturing the animals, he said.

When the move happens, it will help the park meet the goals of the Interagency Bison Management Plan in removing 400 of the animals from the park's growing population.

Yellowstone was estimated to be home to 4,200 bison last summer. The Interagency Bison Management Plan has been used to manage Yellowstone bison since 2000.

Hunts held outside the park's boundaries have achieved nearly half that goal of 400, with some tribal bison hunts still happening, Hottle said.

The 63 bison to be transferred will be part of a contraception program that seeks to reduce brucellosis. The program may be wrapping up in another year, making future transfers uncertain, Hottle said.

So officials are discussing other ways of reaching that population-reduction goal, including the possibility of sending some bison to slaughter, he said.

"Right now, we're looking at that as we speak," Hottle said.

Former Gov. Brian Schweitzer in 2010 blocked a bison slaughter by barring a shipment of the animals through Montana. The federal government is prohibited from transferring bison through the state without prior approval.

It was unclear whether Bullock would continue to block the federal government from transporting bison destined for slaughter.

Montana wildlife officials last year transferred more than 60 bison to the Fort Peck Indian Reservation as part of an effort to relocate more of the animals that once populated the Great Plains to reservations and public lands. Ranchers fought the move in a lawsuit.

Brucellosis, a disease that can cause pregnant animals to abort their young, is one of the major concerns of Montana ranchers seeking to protect their livestock from the spread of the disease.

Some Montana lawmakers are seeking a complete prohibition on transferring bison from the park and they want to loosen hunting restrictions to kill more of the animals.

Bullock's office said in a statement Wednesday the movement of the 63 bison is consistent with the Interagency Bison Management Plan.